Fuel efficiency in cars has become a critical issue particularly due to increase in environmental concerns, stricter legislation and rise in oil prices. One way to enhance efficiency is to reduce fuel consumption. This may be achieved by reducing car component weight.
One way to reduce weight is to replace metal parts of the exhaust system of a car by lightweight composite materials, such as fiber reinforced resin composites. However, composite materials are often not stable up to high temperatures. Since exhaust gases may reach temperatures of 800 to 1000 degrees Celsius, these composite materials need thermal protection.
In U.S. Pat. No. 5,400,830 it is suggested to construct an automotive exhaust pipe by providing a thermal insulation layer inside an outer tube made of a composite material. The tube is provided by a thin-walled metallic inner liner to protect said insulation layer from abrasion by exhaust gases guided inside the pipe. However, metallic tubes still add to the total weight and have low performance in sound absorption.
In order to further reduce weight of the exhaust system, it is suggested to replace the metallic inner tube by a metallic mesh. A metallic mesh holds in place the insulation layer and improves the acoustic properties of the pipe. Often, an insulation layer is a high temperature wool or fiber mat, which fibers are prone to be eroded by the exhaust gas through the openings in the metallic mesh. An eroded insulation has lower thermal insulation properties and may lead to the rupture or the melting of the outer composite tube. Long term stability of such an exhaust pipe is not given.
There is a need for lightweight exhaust elements that have good acoustic performance. Especially, there is a need for exhaust elements, wherein metal parts are replaced by composite materials and that provide equal or better acoustic performance.